The present invention relates to a nozzle box for a steam turbine for directing steam flow from a generally circumferential direction to a generally axial direction for flow through nozzles and particularly relates to a nozzle box having bridges tangentially leaned to match the angles of the leading edges of the partitions.
In steam turbines, nozzle boxes are provided for receiving a flow of steam and directing the steam through first stage nozzles. A nozzle box typically comprises a torus portion having one or more, generally four, inlets for receiving steam, a bridging portion for facilitating a change in the steam flow from circumferential to generally axial directions, and finally, a nozzle ring portion containing partitions defining nozzles for directing the steam flow onto the buckets of the rotor. Nozzle boxes are typically formed in nozzle box halves arranged in a 180° arc, the nozzle box halves having mating horizontal joints to form a continuous 360° nozzle. Conventional nozzle boxes, for example those disclosed in U.S. Pat. Nos. 6,631,858; 6,196,793; and 5,392,513 are representative examples of the foregoing arrangement. For example, as illustrated in U.S. Pat. No. 6,631,858, the bridge ring portion includes a plurality of bridges axially upstream from the nozzle partitions. It will be appreciated that because of structural concerns requiring a substantial number of bridges, the conventional bridge ring portion with multiple bridges has a tendency to restrict the steam entering the nozzles. The bridges, of course, are used to strengthen the nozzle box as well as to straighten the flow. However, conventional bridges afford substantial passage area loss and are generally not matched with the partitions. Accordingly, there is a need for a nozzle box which can efficiently straighten the steam path and reduce the passage area loss.